The use of fuels in various forms generates huge quantities of carbon dioxide, a green house gasand reducing its emission has been accorded top priority in today’s research to protect our climate. The biggest challenge is how to effectively and efficiently capture carbon dioxide anddevelop industrial scale process with lowest possible cost. In the present work we report studieson adsorptive removal of carbon dioxide using various adsorbents which include mainly activatedcarbons- with and without modification. The effect of activation temperature has been studied indetail. Temperature programmed desorption studies have been carried out which clearly indicateshift in the mechanism of sorption on activated carbon surfaces with changes in the activation of carbon and it also depends on the nature of surface/ modification. An attempt has been made toexplain the experimental results of adsorption and temperature programmed desorption usingvarious surface characterization techniques such as surface area, pore size and size distribution,IR and XRD. The results of this work would help in understanding surface interactions duringadsorption/desorption and also in enhancing capacity of carbon dioxide sorption.

Introduction

The use of fuels for generation of energy has a major contribution in the release of green housegas, carbon dioxide. A recent report by Nobel Prize winning Intergovernmental Panel on ClimateChange concluded that global carbon dioxide emission must be reduced to the order of 50-80% by year 2050, if we have to avoid damage to the climate.As far as current state of knowledge is concerned, there is no effective carbon dioxide capturetechnology, as yet, which is not cost and energy intensive (Yue et al., 2008). Thus, the biggestchallenge today is how to effectively and efficiently capture carbon dioxide and developindustrial scale process with lowest possible cost. This, in effect requires research to be carriedout on various fronts involving substantial investment in R&D for identifying and developingmost appropriate processes for carbon dioxide removal. Since burning of fuels can not be avoidedin view of our energy requirements and thus generation of green house gases is also inevitable, it becomes imperative to develop technologies which have potential for reducing green house gasemissions.The processes for removal of gases through adsorption, such as pressure swing adsorption canvery well be used for removal of carbon dioxide (Yong et al., 2002; Liu et al., 2007). The processhas advantage of being cyclic in nature and carbon dioxide is adsorbed at high pressures anddesorbed by lowering the pressure. The process is less capital intensive and also requires lower operating cost. Thermal swing adsorptive process is another alternative. However, primary

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requirement of any adsorption separation is that the adsorbent material should have high capacityfor removal and ease of regeneration. As far as carbon dioxide removal is concerned, this has been a major impediment for application at a commercial level. Song et al. (1998) and Yong et al.(2001) have reported some studies on modified and unmodified carbons and have found thatalthough carbon adsorbents have good capacity for CO

2

at low pressures and low temperatures,the capacity decreases with increase in temperature. They have also suggested that chemicalmodification of carbons can enhance the capacity especially at high temperatures. In view of this,the objective of this work is to evaluate the adsorption/desorption behaviour of variousadsorbents, both modified and unmodified and to study effect of various parameters of adsorption/ desorption characteristics. The Temperature Programmed Desorption (TPD) studiesof this work can provide more insight into the sorption behaviour on the various surfaces andnature of surface interactions. This is expected to further enhance our knowledgebase for suitablymodifying adsorbent surfaces and thereby in developing tailor made adsorbents.

Experimental

Two different adsorbents were used in this work from activated charcoal family. The unmodifiedactivated charcoal was procured from Fluka U.S.A., while the other activated charcoal, acidwashed with phosphoric acid and sulfuric acid was procured from Sigma, U. S. A.. Both thesamples were in powder form and were used without any further treatment/modification. For studying the effect of activation temperature, activation of the samples was done in situ beforeadsorption experiment under constant Helium flow. The activation time was 10 h. After theactivation, the sample was allowed to cool down to room temperature under He flow and thenadsorption of CO

2

was studied.Carbon dioxide adsorption was characterized by temperature programmed desorptionexperiments (TPD). 20 to 30 mg of sample was placed in a quartz tube. After degassing thesample, activation was carried out using the procedure described above. The adsorption of carbondioxide was carried out at room temperature of 30

0

C for 4 h. The adsorbent bed was then flushedwith helium for about 30 min. The TPD tests were then carried out by heating the sample at 10

0

C/min up to 500

0

C with constant helium flowing through the tube and desorbed carbon dioxidewas analyzed using CO

2

detector. The activation temperature effect was studied in the range 100

0

C to 400

0

C and adsorption without activation was also compared. All the gases used in the study(Helium and carbon dioxide) were extra pure above 99.99 % purity.

Characterization of adsorbents

The specific surface area, pore size distribution and pore volume were determined on the basis of nitrogen adsorption using Quantachrome Autosorb-1 Instrument (Quantachrome Inc., USA). X-ray diffraction patterns of the samples were recorded in XPERT-PRO X-ray Diffractometer fromPANalytical Instruments using CuK

α

radiation. The IR spectra of the samples were taken indiffused reflectance mode in Thermo Nicolet FTIR spectrophotometer.

Results and Discussion

Carbon based adsorbents generally have good capacity for adsorption of carbon dioxide,especially at ambient temperatures and low pressures (Yong et al. 2001, 2002). The adsorptioncapacity decreases with increase in the temperature. However, the adsorption capacity at higher temperatures can be greatly increased by suitable chemical modification. This is mainly becauseof the fact that CO

2

is slightly acidic gas and therefore is likely to have more, stronger adsorption

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on the basic sites with increased amounts. Thus, chemical modification to increase the basicity of the surface can have better adsorption characteristics than unmodified carbon adsorbents. Also,adsorption/ desorption mechanism at high temperatures involve more chemisorption for carbondioxide than mere physical adsorption. This is evident from the fact that surface area of thecarbon adsorbents has less effect on sorption capacity at high temperatures clearly indicatingmore surface interactions than pure physical adsorption. Similarly, it is expected that activation of the material using different temperature can also affect the adsorption/ desorption behavior of carbon dioxide, through modification of surfaces/ adsorption sites.The results of three different activation temperatures for the unmodified activated charcoal areshown in Figure 1 for activation temperatures of 100, 200 and 300

0

C respectively. The TPD plotsclearly show substantial difference in the sorption characteristics in these three cases. For activation temperature of 100

0

C, two peaks were observed in the TPD. The first desorptionobserved at 80

0

C and the peak was observed in the range 80 – 140

0

C, while the 2

nd

desorption peak was observed in the temperature range of 180-330

0

C. The second peak was much larger compared to the first. The occurrence of the two peaks here indicates difference in the nature of chemisorption and also possibility of macropore/micropore desorption taking place. For theactivation temperature of 200

0

C, the similar two peaks were observed. However, here there is ashift in the desorption temperature and also in the temperature range of desorption. The first peak in this case is much smaller compared to that observed with 100 C activation temperature and can be seen to initiate at ~100

0

C and the 2

nd

peak at ~260

0

C. The 2

nd

peak is much broader and isobserved in the range 260 – 430

0

C. The change in the TPD behaviour clearly points to themodification in the sorption sites with activation temperature. The TPD plot of activationtemperature 300

0

C is also shown in Figure-1 again clearly demonstrates the shift in thedesorption temperature and sorption behaviour as described earlier. In order to further confirm